Streptavidin is a nonglycosylated, tetrameric protein, with each subunit able to bind a single molecule of the vitamin biotin. Streptavidin-biotin bond is the strongest known non-covalent interaction with Kd ~10-15 M. Because streptavidin lacks any carbohydrate modification and has a near-neutral pI, it has the advantage of much lower nonspecific binding than avidin. Streptavidin-biotin interaction is widely used in experimental biology for purification and detection of biomolecules.